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URLClassLoader not loading jar file

 原文  2019-03-04 15:53:56  3   1    java

Question

I'm currently working on an web application, that must load jar files from a directory on the local drive. After a bit of researching, the URLClassLoaders seems to be the best fitting solution.

The beef: 

public @Slf4j class ActionLoader {
private static void addFilesToClassPath(File path) {
        URLClassLoader clazzLoader = null;
        try {
            clazzLoader = new URLClassLoader(new URL[] {path.toURI().toURL()},
                                         ActionLoader.class.getClassLoader());
            try {
                Class.forName("de.befoe.xyz.ActionTest", false, clazzLoader);
            } catch (ClassNotFoundException e) {
                log.error("Class not found", e);
            }
            try {
                clazzLoader.loadClass("de.befoe.xyz.ActionTest");
            } catch (ClassNotFoundException e) {
                log.error("Class not found", e);
            }
        } catch (SecurityException e) {
            log.error("Access denied", e);
        } catch (MalformedURLException e) {
            log.error("Malformed URL", e);
        } finally {
            if (clazzLoader != null) {
                try {
                    clazzLoader.close();
                } catch (IOException e) {
                    log.error("Error closing the class loader", e);
                }
            }
        }
    }
}

First ClassNotFoundException : 

java.lang.ClassNotFoundException: de.befoe.xyz.ActionTest
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at de.procrm.speedindexer.scr.ActionLoader.addFilesToClassPath(ActionLoader.java:80)
    at de.procrm.speedindexer.scr.ActionLoader.loadActions(ActionLoader.java:31)
    at de.procrm.speedindexer.scr.Init.contextInitialized(Init.java:39)
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4842)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5303)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147)
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:725)
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:701)
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:717)
    at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:940)
    at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:467)
    at org.apache.catalina.startup.HostConfig.check(HostConfig.java:1612)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.apache.tomcat.util.modeler.BaseModelMBean.invoke(BaseModelMBean.java:300)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:819)
    at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:801)
    at org.apache.catalina.manager.ManagerServlet.check(ManagerServlet.java:1451)
    at org.apache.catalina.manager.ManagerServlet.deploy(ManagerServlet.java:724)
    at org.apache.catalina.manager.ManagerServlet.doPut(ManagerServlet.java:423)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:651)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:108)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:614)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1099)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:670)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)

Second ClassNotFoundException : 

java.lang.ClassNotFoundException: de.befoe.xyz.ActionTest
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:348)
    at de.procrm.speedindexer.scr.ActionLoader.addFilesToClassPath(ActionLoader.java:81)
    at de.procrm.speedindexer.scr.ActionLoader.loadActions(ActionLoader.java:31)
    at de.procrm.speedindexer.scr.Init.contextInitialized(Init.java:39)
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4842)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5303)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147)
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:725)
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:701)
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:717)
    at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:940)
    at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:467)
    at org.apache.catalina.startup.HostConfig.check(HostConfig.java:1612)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.apache.tomcat.util.modeler.BaseModelMBean.invoke(BaseModelMBean.java:300)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:819)
    at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:801)
    at org.apache.catalina.manager.ManagerServlet.check(ManagerServlet.java:1451)
    at org.apache.catalina.manager.ManagerServlet.deploy(ManagerServlet.java:724)
    at org.apache.catalina.manager.ManagerServlet.doPut(ManagerServlet.java:423)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:651)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:108)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:614)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1099)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:670)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)

The path-variable points to the directory containing the jar files (I also tried initializing the class loader with URLs pointing to the jar files directly).

When I now try to access a class from one of the jar files, I get a ClassNotFoundException . When inspecting the class loader in the debugger, I can also see, that the packages- and the classes-fields are both empty, so it doesn't seem as the class loader did load anything.

First I thought it has something to do with missing dependencies, so I created a second jar file with no dependencies and added the classes parent class loader to the constructor, but it changed nothing.

Ideas?

E: After a bit of researching it seems my first thought (dependencies) was correct. I have another library that is used by the web application and the loaded jar files. If I add this library to the the list of jars to add to the URLClassLoader, the jars are loaded just fine. But sure, this is not, what I want to do, because technically my web app uses another class than my jar files, what leads to other problems.

The question now is why the parent class loaders classes are not used properly. After further research it seems the parent class loader I used by calling ActionLoader.class.getClassLoader() was not the correct one. In eclipse debugger therefore I saw a field called delegate which was set to false. So I instead use this class loaders parent, and the files are loading just fine.

1 answers

solution1
 0  2019-03-04 16:22:23

I don't have your JAR file so i've tested with one of my JAR files on a custom path, and the class is correctly loaded, using the systemClassLoader.

Here is the sample code: 

addFilesToClassPath(new File("C:/Users/Domenico/.m2/repository/com/doctorchat/core/0.1.ALPHA/core-0.1.ALPHA.jar"));

private static void addFilesToClassPath(File path) {

   try(URLClassLoader clazzLoader = new URLClassLoader(new URL[]{path.toURI().toURL()}, ClassLoader.getSystemClassLoader())) {
            Class c = clazzLoader.loadClass("com.doctorchat.core.exceptions.NotImplementedException");
            System.out.println(c);
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        //  clazzLoader.getResource()

    }

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